Phototube



Fgb. 17, 1942. A. M. GLOVER PHOTOTUBE Filed Sept. 29, 1939 fl w ATTORNEY.

, absorb the" alkali m vaporized at will f" Patented Feb 17, 1942 UNITED 'P oI fF rcE a rno'roriiim han oiavs aa' i Qrafige, *1 a sig i a ita o Coipoi'ation ofAmericam'corporation of "Delaware I gagiiieaticrg's'e iteniber 29, late, seamstress I v @Claim S. 31. 250- 165) ini'rerition rlates to [photo-electric :tiib'es andlparticularly to siich tubes sensitized 'wi thalrm r mdabn' of a photo-emissive surface in which caesium or other alkali metal is apipl'ie'd to fan oxidized metal surface may involve 1 hi nc i rthe. i'uhep a n e a ua ti r or 'e'xcessfalka li metal atteithe desired prioty is obtained; 'Ihis excess is -de sire tor variofuisj i'easons and also because t1}; yield of alkali metal from the the art areinevitable, and contaway-same excess alkali 'metalmust be :present as a safety .factor in the sensitizing process. 1 A The'dornpleted tii' be should be free from excess alitfali meta 1.; Therefore alkali metal absorbents, such-as eopper o gide, lead oxide and tin oxide are generally used. Withsu'oh 'absorbents, which are active whenever the --alka'l-i meta-l vapor is present the-tubafconsiderab1e alkali fmetal is lostto the absorbent during the sensitizing step, "and the treatment; and retreatment if necessary, of the cathode fwith caesium is not under as good conesteem is --uncontrol-lable.'

it -is an object of my invention to provide a 'phototub'e which may be manufactured with the sensitizing -;s'teps during the manufacture 'u'rider tr l h s desirable, since the action of the ab- I easy and-accurate control. "It is "another object s the course or hia'nuiacture of the 5 to provide a -phototube which, if incorrectly processed, may be easily reprocessed under controlled' conditions "to obtain optimum results. it is a further object of my invention to provide a phototube wherein all excess or free alkali metal may be absorbed after the cathode is "treated byasin'gl'e step or after repeated steps of' vaporizing alkali metal without dang'er ojf the absorber becoming impaired or saturatedduring oath-ode treatment; Y

In accordance with my invention, 1" provide within an evacuated envelope containing axial- ,lcali 'n-ietaltreatedphotosensitive surface a source 1 pnotq'sefis'aivat. By exegs'sjaikan metal I mean all alkali is not'chernio ally or physically w 91 t e bhjetee i es rf a d w i is free toievap oi ate at the evaporation temperatiire bf hulkimaterial.

A better understandingof invention will be obtamed and other objects features and ad'- 'e e es wil reaa r t following ir ntion takenin connection with the accompanying drawing wherein the single figure is a longitudinal v ew partially in section of a phototube emdrme'm in nt n, I

Referring to the drawing, I provide an enyelope or bulb l enclosing an electron emissive ori qll otosensitive cathode 2 which in the construction shown is preferably of the arcuate type, V

ar d 'a-n anode 3 exposed to the photosensitive surface of the photocathode 2 so that it may receive electronswhich are liberated from the photocathode under the influence of light. The photocathode 2 and anode} are preferably sup:

- ported by a pinchor press 4 in which are sealed of niaterial whi'h in massive form i's 'substaniti'ally inactive with respect to the alkali metal hilt which iii a t n, clean layer or film will ,tal and combine with it to elean up allia metal and render it ineffective. More partioul' h I provide a mass or antimony, arsenic or bismuth;

e 1 as g 'te ijqn 9f caesium or other alkali metal a phototube toforrh a film absorbs the excess alkali ih'etal. In this was th flii riirlati'dn of thevef flct j of the free alkali metal may be contrblie'd very edge or which Ifnay be theyeurrent carrying leads to the cathode and anode. ,The tube shown in the drawing is provided with a tribulation 5 through which the tube is evacuated, thistubulation later being sealed "following the sensitization process.

A source of caesium or other alkali metal either within or without the bulb I may be provided, but I prefer to provide a quantity of caesium-bearing compound such as' the activator pellet 6 which is in good thermal 'contact with a metal tab 1 r so that caesium may be liberated from the pellet by suitable heat treatment. x

In accordance with my invention, I provide an excess alkali absorber such as a mass of vapori- "zable material which may be vaporized and deposited as a thin filni 9 on the inner wall of the bulb l at some point remote from the oatho'de and exit of contact with the press 4 and "cathode 2. a a

' The ex ess :alkali absorber is selected from the rritalloiols arsenic, antimony and bismuth,

although I prefer antimony, which is readily available in massive form and which, when vaporized, forms upon the bulb wall a good film or layer with a metallic luster. In describing the preferred form of my invention I will specifically refer to antimony as the absorber, but it is to be understood that my invention is not limited to antimony, as arsenic or bismuth may be used instead of antimony.

The photocathode 2, preferably of a metal such as silver with an oxidized surface, is activated with alkali metal in the conventional way. Following the evacuation of the bulb I, oxygen is admitted to the bulb I through the tubulation 5 and the silver cathode is oxidized by a glow discharge in oxygen between the cathode and anode. For example, the cathode may be oxidized until a bright green color following a bright red is obtained. The oxygen is then removed, the bulb l is highly evacuated, and the activator pellet 6 is heated to liberate alkali metal vapor within the envelope. I have obtained good results and high photosensitivity with a cathode having an area of approximately 1.5 sq. inches using a caesium activator pellet 6 consisting of 45 to 50 mg. of a mixture of one part caesium dichromate by Weight to two parts of silicon. This pellet contains a great excess of caesium over that theoretically required for the proper activation of the cathode, for the reason that the quantity of caesium liberated should be in excess of that required for optimum photosensitivity, and the amount of this excess will vary with the yield of the caesium-bearing activator. The activator pellet is preferably heated and caesium vaporized while the tube is at approximately room temperature. Following the vaporization of the caesium, the tube is baked at a temperature of 265 to 285 C. until the cathode turns to a gold or straw color, whereupon the phot'ocathode will be found to have high photosensitivity. At the same time, high reverse current leakage may be present between the cathode and anode which indicates the presence of excess caesium within the tube.

The tube is freed of excess alkali metal, either before or after sealing off, by heating the excess alkali metal absorber 8 to vaporize enough of the antimony or similar material to form on the inner wall of the bulb a bright film 9 of metallic appearance and having an area greatly exceeding that of the absorber 8. The speed of the reaction with the excess alkali metal may be increased by maintaining the envelope or bulb l at a temperature of approximately 150 C. during the vaporization of the absorber. For convenience in vaporizing the absorbent at will I prefer tomount a mass of antimony within or on a helically coiled heater filament l0 made of metal having a melting point higher than the vaporization temperature of the absorber 8. To limit the film 9 to the bulb'wall and to prevent the absorbent from becoming deposited either on the press 4 or on the cathode 2 I may use a shield member II which surrounds the filament [0 which is mounted directly on a current-carrying lead [2 sealed in the press 4. One end of the filament is connected to the conductor I2 and the other end to the anode 3 in order that current may be passed through the filament to heat and vaporize the antimony at will. The amount of the absorbent such as antimony either on or within the helical filament I0 is large in comparison with the amount normally needed to absorb the excess caesium, so that if for any reason additional absorbent is required, a source within the tube will be available. If a measurement of the photosensitivity indicates that an optimum has not been reached and that the photosensitivity is lower than desired, additional caesium may be vaporized from the activator pellet 6, followed by rebaking of the tube. The tube is then freed of the excess or free alkali metal thus introduced by again heating the absorber 8 and vaporizing enough antimony or similar absorbent to form a new clean, active film 0n the inner wall of the bulb.

Little or no caesium is absorbed by the mass of material comprising the absorber 8 during the liberation of caesium from the activator pellet 6 even at room temperature, thus permitting the proper formation of the photosensitive surface. The'absorbent, either while it is in the vapor stage or immediately after it has been deposited as the film 9, reacts or combines with the caesium or similar alkali metal to convert the alkali metal to a stable product, probably an alloy, which vaporizes with difficulty, so that the excess caesium is rendered ineffective and harmless during operation of the tube.

I have found that metals such as silver do not react with the excess alkali metal to absorb or fix the alkali metal in the same way as antimony, arsenic and bismuth. For example, silver has been evaporated within the tube subsequent to the sensitization of the cathode to form a film 0n the bulb wall, but leakage measurements indicated substantially no decrease in the quantity of excess caesium following the vaporization of silver.

While I do not wish to be limited to any particular theory to explain the improved results obtained by following the teachings of my invention, it appears that when antimony in the vapor stage is intimately exposed to any excess alkali metal within the envelope a very stable alloy of antimony and the alkali metal is formed, from which the alakli metal will not be released during operation of the tube. Therefore, phototubes made in accordance with my invention may be operated at elevated temperatures without danger of liberating alkali metal from the film of absorbent, whereas many prior tubes using metallic oxides or carbon as a caesium absorbent must be designed to operate at temperatures approximating normal room temperature.

The complete lack of leakage due to the fixing or absorption of excess alkali metal makes my process particularly applicable to the manufacture of television transmitting tubes using either a continuous photocathode or a target electrode of the mosaic type having a multiplicity of silver particles. My process is well adapted for use in the manufacture of such mosaic electrodes wherein the silver particles of the electrode are oxidized, caesium admitted to the tube and the excess caesium absorbed by vaporizing a mass of my alkali metal absorber within the envelope. One of the main problems of forming a photoelectric mosaic under present methods is the elimination of caesium from between the particles. In accordance with my invention, this excess alkali metal deposited between the mosaic particles is absorbed, thereby reducing the electrical leakage between the particles.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.-

I claim:

1. A photo-electric tube comprising an envelope enclosing an anode, a cathode photosensitized with alkali metal, and a thin layer comprising an alkali metal and a metallic element selected from the group consisting of antimony, arsenic and bismuth on a portion of the inner wall of said envelope and disposed remote from said cathode.

2. A photo-electric tube comprising an envelope, a cathode, a photosensitive coating of alkali metal on said cathode, an anode exposed to said cathode, a mass of metallic material selected from the group consisting of antimony,

arsenic and bismuth mounted within said envelope and a film including a metal selected from the said group of metals on the inner surface of said envelope adjacent said quantity of metal said film also including an amount of alkali metal absorbed from the atmosphere of said envelope.

3. A photo-electric tube comprising an evacuated envelope, a cathode having a photosensitive coating of alkali metal, an anode exposed to the coating of alkali metal on said cathode, a mass of antimony within said envelope adapted to be vaporized within said envelope and a film of alkali metal-antimony alloy on a surface Within said envelope disposed remote from said cathode.

4. A photo-electric tube comprising an envelope, a caesium-treated photosensitive cathode within said envelope, an anode in electron receiving relation with respect to said cathode, a mass of antimony within said envelope adapted to be vaporized within said envelope and a composite caesium-antimony film on a surface within said envelope remote from said cathode and removed from the direct path between said cathode and anode.

ALAN M. GLOVER. 

